1. Field of the Invention
This invention relates to an image forming process for forming full color images of quality by subjecting full color photosensitive material to scanning exposure using light sources modulated in accordance with image data.
2. Prior Art
For photographic material using silver halide as a photosensitive element which is generally known as photographic silver halide photosensitive material, it is well known to form images by modulating laser light in accordance with recording signals, and scanning the photosensitive material with the modulated light. This system allows full color images of high precision to be produced using digital image information. Further the recent development of laser diodes has led to the manufacture of simple stable laser output devices. Image forming systems based on these technologies are expected. For example, U.S. Pat. Nos. 4,619,892 and 4,956,702 disclose color photographic silver halide photosensitive materials having sensitivities in the infrared region that can be exposed by beams emitted from laser diodes.
These processes of forming images using signals which are controlled in accordance with digital image information allow for synthesis of a plurality of image information bits and processing of an original image information bit in various ways, spreading their use to a wider variety of applications beyond the limit of the image forming process based on the conventional photographic technology.
The photosensitive material used as an image output medium involves a color reproduction mechanism based on the subtractive color process using yellow, magenta and cyan dyes as the three primary colors like the conventional color photographic photosensitive material. Since exposure for digitally controlling the generation of the three primary colors can be done independently for each color, color generation can be controlled at a higher degree of freedom than the conventional color photography using color negative and color paper.
In particular, laser light sources including semiconductor lasers such as laser diodes and gas lasers emit light beams having a narrow band of wavelength. Then the use of laser light sources has some advantages. Even when a color photosensitive material having wavelength-dependent spectral sensitivity is exposed, for example, the three primary colors can be independently generated by using a plurality of (three in this case) laser light sources emitting laser beams having different narrow band wavelengths. Also when a color photosensitive material which has overlapping tails of adjacent spectral sensitivity curves and which insufficiently separates color is exposed with the laser beam of a semiconductor laser whose oscillation wavelength can vary to a wavelength region causing color mixing, it is proposed to electrically process image signals so as to prevent color mixing and to increase color purity. Also proposed are photosensitive materials featuring better separation of the three primary colors, which provide more freedom to the independent control on the generation of the three primary colors. Then in the case of digital exposure, some provisions for increasing the saturation of a color to be reproduced, for example, can be made in delivering original image information outputs. In practice, several image processing techniques have been attempted.
The method for carrying out digital exposure on photosensitive material using semiconductor laser includes a light intensity modulation mode of modulating the intensity of laser light in accordance with the density of an image and a time duration (pulse width) modulation mode of modulating the light emitting time in accordance with the density of an image. The light intensity modulation mode involves delivering light intensity signals corresponding to the image density as modulation signals, and increasing or decreasing the current flow applied to the semiconductor laser in accordance therewith to adjust the output of laser light per unit time, thereby changing the exposure quantity for forming a graded image. The time duration modulation mode involves delivering time duration signals corresponding to the image density as modulation signals, and changing the duration of conduction of a semiconductor laser to increase or decrease the output time duration, thereby changing the exposure quantity for forming a graded image.
Output signals for modulating an exposure light source which may either be light intensity modulating signals or pulse width modulating signals are obtained from original image data by performing computation using the following conversion formulae: EQU Ce=f(Ro, Go, Bo), EQU Me=g(Ro, Go, Bo), and EQU Ye=h(Ro, Go, Bo)
wherein Ro, Go, and Bo represent red (R) , green (G) , and blue (B) signals of the original image data, respectively, and Ce, Me, and Ye represent output signals for exposure light source control corresponding to the cyan, magenta and yellow dye-forming layers, respectively.
For the purpose of increasing the purity of a reddish color which is to be reproduced, conversion in accordance with the following expression: EQU Ce=Ce[1-.alpha.(Ye+Me)]
may also be utilized. The turbidity of the reddish color can be reduced by setting the value of .alpha. to an appropriate positive number.
In practice, the computation formulae can be varied due to various properties of the photosensitive material to be used as an output medium, for example, the absorption properties a color developing dye and sensitivity. Then in designing a particular image forming system, these computation formulae must be determined from the properties of the equipment and photosensitive material used. The characteristics of the functions f,g, and h and actual values of .alpha. may be determined, for example, by performing color matching between the original image and an output image for several tens to several thousands of colors in accordance with the method of least squares.
Formation of full color images using photographic silver halide photosensitive material enables high density recording as compared with other methods such as dye sublimation transfer, electrophotography and ink-jet printing methods, resulting in images being improved in such quality factors as granularity, sharpness and gradation reproduction. Therefore, by writing in image information according to this method, images of high quality which could otherwise be established only in the analog system as represented by photography can be obtained from digital signals as hard copies.
Investigating the system for writing in images in photographic silver halide photosensitive material, especially featuring improved color separation, by scanning exposure using a laser light source modulated in accordance with digital image information, we found an unexpected problem in reproducing original images. That is, the delicate graded depiction of highly color generated areas of an image having high saturation varies, and it is thus difficult to obtain a stable image output. More particularly, in reproducing a reddish image having high saturation, for example a crimson rose and a red velvet dress, there occurs a phenomenon that the depiction of delicate shades varies, leaving a serious obstruction in reproducing an image of quality in a stable manner.
In general, in color reproduction by the subtractive color process, delicate shades in areas having high degrees of saturation and color generation are represented by the ratio of that color to a color complementary thereto. With only those colors having high purity, depiction is saturated within the range between minimum and maximum degrees of color generation so that only a narrow range can be depicted. For example, shades in a high density area of high purity red are depicted by a change in the color generation proportion of a cyan component starting from near the highest density of red color generation. As the proportion of a cyan component varies from the minimum to the highest density of color generation, the color changes from highly pure red to achromatic black.
It is therefore understood that in order to consistently express delicate shades in highly color generated areas of highly pure red, the color generation of a low density cyan component responsible for their reproduction must be controlled as precisely as possible. Differently stated, a variation in color generation of such a low density component is presumed to cause inconsistency in the reproduction of delicate shades as previously described.
This problem becomes more outstanding in those shaded areas having high degrees of saturation and color generation among the original image information. This is because the control of a light source is not carried out in a fully stable manner in those portions in which output signals for modulating an exposure light source in accordance with the image signal are of small values.
Semiconductor lasers such as laser diodes have characteristic curves of their light output which include high and low light output regions in which the characteristic curve of light output is linear and non-linear relative to the input signal level, respectively. The semiconductor laser is not sufficiently stable to accurately control the color generation density of cyan, for example, for depicting delicate shades in a low light output or non-linear region, thus failing to achieve good gradation. Also, if the level of light output is further reduced, no laser oscillation occurs such that an output region for light emitting diodes and optical elements for laser beams cannot provide full control. In contrast, even when a complementary color is generated for the depiction of delicate shades by pulse width modulation using a light output in the linear region of this laser, the pulse width is significantly reduced, stable control becomes difficult, and good graded depiction is not available.
On the other hand, in order to adequately adjust the exposure quantity of a light source in proportion to a density, it is contemplated to logarithmically vary the exposure quantity, which in turn, may be achieved by logarithmically varying the value of modulation control output signals for setting the intensity or time duration of laser beams or other light beams. Since it is, however, very difficult to construct such a control circuit, a general practice is to anti-logarithmically vary a control quantity in an arithmetic manner. For example, when the modulation of a light source is controlled using a micro-computer, an equal number of modulation control signals to the number of combinations of bits in one byte are obtained so that a constant change of the output is available per modulation control signal.
As a result, as shown by solid lines in FIG. 4, in a region having an increased exposure quantity (and hence an increased image density), a change .DELTA.logE of the logarithmic value of an exposure quantity corresponding to a change .DELTA.S of a modulation control signal is small and the corresponding differential density .DELTA.D.sub.1 is small. in a region having a reduced exposure quantity (and hence a reduced image density), a change .DELTA.logE of the logarithmic value of an exposure quantity corresponding to the change .DELTA.S of the modulation control signal as above is large and the corresponding differential density .DELTA.D.sub.2 is very large. Accordingly, there is a problem that if the visual threshold is exceeded, a density difference or density leap .DELTA.D.sub.2 occurs in a low density region, failing to achieve good gradation and lowering reproduction ability.
Therefore, such a modulation control system is difficult to achieve color generation at a sufficiently low density to depict delicate shades and fails to achieve stable gradation. There is an additional problem that a density variation in shades becomes more outstanding with a lower density of a complementary color added as the shades. It was also found that the magnitude of such a variation is further increased if the signal processing for increasing the saturation of a pure color as mentioned above is employed.
U.S. Pat. Nos. 4,619,892 and 4,956,702, referred to above, describe the construction of exemplary laser writing photosensitive materials by combining color couplers capable of forming yellow, magenta and cyan dyes through a coupling reaction with an oxidized form of an aromatic primary amine developing agent with a silver chlorobromide emulsion which is spectrally sensitized in an infrared region. The techniques disclosed in these patents produce full color images by sensitizing at least two photosensitive layers in the infrared region and subjecting them to scanning exposure by means of a semiconductor laser. According to their teaching, the serious problem of deteriorated color separation induced by a broad spectral sensitivity distribution resulting from infrared sensitization is solved by providing a differential sensitivity or a filter layer between the photosensitive layers. Therefore, what is disclosed by the patents is to provide a color having as high as possible purity. Such a method, is difficult to achieve consistent reproduction of delicate shades in areas having high degrees of purity and color generation as previously described.
Regarding conventional color photographic silver halide photosensitive material, U.S. Pat. No. 4,806,460 discloses the use of a photosensitive material in which for the purpose of increasing the reproduction latitude of a color of high purity, in a particular image region in which the image density of one dye is at or above a certain value set between 1.2 and 2.5, one dye having a hue which does not substantially contribute to formation of the hue of the particular image is added while providing gradation. However, this technique is to enhance the shades in colors of high purity in prints resulting from conventional negative films and thus completely different from the technique of eliminating a variation in the depiction of graded detail which becomes a problem in writing using a light source modulated using image data as in the present invention.
This is explained as follows. If it is only desired to depict shades in colors of high purity, it is naturally presumed that the writing system using a plurality of light sources which are independently modulated in accordance with image data has the flexibility of changing the magnitude of modulation. Therefore, the problem to be solved by the present invention, the occurrence of a variation in the reproduction of shades in highly color generated portions of a high saturation color is a new problem which is first recognized when writing from image data is performed.
Moreover, the technique of U.S. Pat. No. 4,806,460 relates to the addition of spectral sensitivity to a region corresponding to a complementary color to the spectral sensitivity distribution of one photosensitive layer and simultaneous color generation of a dye of high purity and a dye corresponding to a complementary color thereto. In this regard too, this patent employs a different technique than the present invention.